Nonmetallic composite pipe, employing reinforcing fibers such as glass, graphite, and Kevlar, and bonded together by a thermosetting resin matrix has been joined together in the past by a wide variety of threaded coupling systems. Fiberglass-reinforced composite pipe, employing threaded couplings, has been used, in particular, in a wide variety of subsurface applications including water-well casing and oil-well tubing.
However, threads cut into fiberglass-reinforced composite pipe cut the reinforcing fibers as well as the resin matrix, thus reducing considerably the structural strength of the composite pipe at the threaded couplings. Consequently, the strength of such threaded couplings is limited by the interlaminar shear strength of the resin matrix rather than the strength of the reinforcing fibers. As a result, the strength of the coupling, in particular, its axial tensil strength, is considerably less than the strength of the pipe body, in particular, its axial tensile strength. Therefore, if a coupling system were designed that was at least as strong as the composite pipe body, the composite pipe, with its equally-strong coupling system could be subjected to much more demanding service, involving much higher tensile loads.
This invention describes a completely new approach to joining together nonmetallic composite pipe by means of a steel coupling system, which is equal in strength to that of the composite pipe body.
The thread system employed to join the two halves of the steel coupling together is of the type used on oil well tubing, and was selected because of the low pitch of the threads, which is compatible with the gasket sealing system and the anti back-out set-screw system. Furthermore, this type of threaded connection is not plagued by joint swelling problems in high-tensile-load and high-temperature subsurface applications involving deep oil and gas wells, and geothermal wells.
The coupling system, unlike the standard oil-well tubing coupling, when completely threaded together, results in an internally-flush central passage. The internally-flush central passage, made possible by the compressible gasket system reduces the probability of fluid-flow irregularities at the couplings, which, otherwise might damage the coupling system as a result of concentrated abrasion or cavitation.
Each half of the steel coupling is connected to longitudinally-reinforced composite pipe by means of the gripping action created by an inner tapered steel friction sleeve, which is pressed, at the time of fabrication, into the end of the composite pipe, thus radially expanding and radially compressing the wall of the composite pipe between the inner tapered steel friction sleeve and the tapered inner surface of the steel half-coupling. Such a friction-grip connection between the steel coupling and the composite pipe wall translates the axial tensile stress across the coupling system into radial compressive stresses exerted upon the composite pipe wall and the inner tapered steel friction sleeve, and into concentric tensile stresses around the internally-tapered steel half-coupling. Consequently, the greater the axial tensile stress across the coupling system the greater is the gripping action created by the radial compression of the composite pipe wall and inner tapered steel friction sleeve as confined and compressed by the internally-tapered surface of the steel half-coupling. Deformations on the outer surface of each inner tapered steel friction sleeve discourage its longitudinal displacement with respect to the longitudinally-reinforced composit pipe whereas deformations on the tapered inner face of each steel half-coupling discourage rotational displacement of each steel half-coupling with respect to the longitudinally-reinforced composite pipe.
Therefore, the friction-grip connection between the nonmetallic composite pipe and the steel coupling system should be equally as strong as the composite pipe body itself since no part of the coupling system depends upon threaded composite pipe and the accompanying weakened condition that otherwise would result from the cut reinforcing fibers.